96 THE TEXTURE OF THE SOIL [chap. 



of soil be combined with these percentages to calculate 

 the thickness of film of the water on that surface, it will 

 be found that on all soils the film possesses approxi- 

 mately the same thickness, about 0-00003 inch. 



Because of the quantity of water which some soils 

 will retain rather than give up to the plant, it is possible 

 that such soils may have less available water for the 

 plant than a much coarser grained soil which starts 

 with a lower initial amount of water. For example, the 

 clay and sand in the table, p. 95, contain when satu- 

 rated about 26 per cent, and 18 per cent, of water 

 respectively ; as the crop can reduce this to 12 per cent. 

 in one case, and 4-2 per cent, in the other, both sand 

 and clay yield about the same amount of water to the 

 crop. 



A good example of the fact that only the water 

 in the soil which is in excess of the moisture held at 

 the wilting point is available for the crop, is seen in 

 F. J. Alway's studies of soil moisture conditions in 

 the " Great Plains " region of north-western America. 

 There the rainfall is only from 12 to 15 inches annually, 

 and falls chiefly during the summer months ; because 

 of its insufficiency for the production of continuous 

 crops, it is customary to take a bare fallow one season 

 in three in order to accumulate the rainfall for the 

 benefit of the two succeeding grain crops. 



The table shows the water content of the soil down 

 to the depth of 6 feet on two fields near Indian Head, 

 Saskatchewan, taken at the end of July 1904, field B 

 having been fallowed, and C having carried a crop of 

 oats which had shown the effects of drought. Figures 

 are given for the total water in the soil as sampled, the 

 hygroscopic moisture as determined in the laboratory, 

 and the difference, which Alway termed the free 

 water : 



